The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
The present invention relates to methods and apparatuses for sensing or detecting structural damage in objects, more particularly to methods and apparatuses for sensing or detecting fractures, delamination and other forms of mechanical damage in structures such as composites.
Presently there is no completely satisfactory method or mature sensing technology to detect composite damage in-situ and in real-time, or to remotely interrogate such occurrences. Sage et al. U.S. Pat. No. 5,905,260 issued May 18, 1999, incorporated herein by reference, disclose a sensing technology to make such measurements; however, the technology disclosed by Sage et al. has its limitations. The sensor proposed by Sage et al. is a discrete sensor (described by Sage et al. as including xe2x80x9ca small piece of a triboluminescent materialxe2x80x9d) that will locate damage only when damage occurs within the sensor""s triboluminescent material itself. This approach by Sage et al. is problematic for various reasons. Firstly, the area (areas) of interest is (are) very specifically targeted according to the Sage et al. methodology, which assumes that the user of the composite structure knows exactly where damage will occur. Secondly, the Sage et al. methodology assumes that any damage in the composite is transferred to cracking in the sensor""s triboluminescent material. Thirdly, measurements made at or near the outside surface of the composite may be impossible to make in accordance with the Sage et al. methodology.
In addition to Sage et al., other United States patents disclose triboluminescence or triboluminescent material in some context or capacity, including the following which are incorporated herein by reference: Qiu et al. U.S. Pat. No. 6,281,617 B1 issued Aug. 18, 2001; Storey U.S. Pat. No. 6,270,117 B1 issued Aug. 7, 2001, Akiyama et al. U.S. Pat. No. 6,159,394 issued Dec. 12, 2000; Watanabe et al. U.S. Pat. No. 6,117,574 issued Sep. 12, 2000; Hall-Goulle U.S. Pat. No. 6,071,632 issued Jun. 6, 2000; Hansma et al. U.S. Pat. No. 5,581,082 issued Dec. 3, 1996; Pappalardo et al. U.S. Pat. No. 4,772,417 issued Sep. 20, 1988; Dante U.S. Pat. No. 4,372,211 issued Feb. 8, 1983; Glass, deceased et al. U.S. Pat. No. 4,020,765 issued May 3, 1977.
xe2x80x9cTriboluminescentxe2x80x9d (sometimes called xe2x80x9cmechanoluminescentxe2x80x9d) material is a substance (usually, a crystalline substance) that, when fractured or otherwise subjected to some form of mechanical action, releases optical radiation in the visible spectrum (about 400-700 nm). Although the consistency of some triboluminescent materials may be roughly compared to that of sand, this is not a valid comparison for many triboluminescent materials. There are numerous different triboluminescent substances which share certain attributes and thus exhibit triboluminescent properties. A triboluminescent substance can be comprised of any of various organic and inorganic materials. As used herein, the terms triboluminescence (or triboluminescent) and mechanoluminescence (or mechanoluminescent) are synonymous. Both terms refer to luminescence (light emission) resulting from mechanical action, such as friction (rubbing), pressure, fracturing, scratching, striking, sawing, crushing, pulverizing, smashing or tearing.
Derivationally, triboluminescence comes from xe2x80x9ctriboxe2x80x9d or the Greek term xe2x80x9ctribein,xe2x80x9d meaning xe2x80x9cfrictionxe2x80x9d or xe2x80x9crubbing.xe2x80x9d Notwithstanding that the prefix xe2x80x9ctriboxe2x80x9d seems to connote a more specific meaning of xe2x80x9cfrictionxe2x80x9d or xe2x80x9crubbing,xe2x80x9d the terms triboluminescence and triboluminescent are intended herein to denote luminescence resulting from any and all forms of mechanical action. The prefix xe2x80x9cmechanoxe2x80x9d in xe2x80x9cmechanoluminescencexe2x80x9d and xe2x80x9cmechanoluminescentxe2x80x9d seems to have a broader connotation more in keeping with the broadest concept of luminescence resulting from any and all forms of mechanical action. Nevertheless, the terms triboluminescence and mechanoluminescence are used fairly interchangeably in conventional usage. Thus, the terms triboluminescence and mechanoluminescence are intended herein to have the same broadest meaning, viz., luminescence resulting from any and all forms of mechanical action. Consistent with conventional usage, the terms fractoluminescence and fractoluminescent are intended herein to more specifically denote luminescence resulting from fracturing, since the prefix xe2x80x9cfractoxe2x80x9d suggests the more specific meaning of xe2x80x9cfracture.xe2x80x9d
In view of the foregoing, it is an object of the present invention to provide method and apparatus for remotely detecting mechanical damage (e.g., fracture or delamination) occurring in a structure of any kind, especially to provide such method and apparatus in relation to a structure which is a composite structure such as a matrix composite or a laminar composite.
It is another object of the present invention to provide method and apparatus for remotely monitoring the mechanical damage condition of a structure which is known to be susceptible to certain types of damage such as involving fracture or delamination.
In accordance with typical practice of the present invention, a combination is provided which is suitable for detecting damage in an object. The combination comprises fiber optic means and triboluminescent means. The fiber optic means and the triboluminescent means are each adaptable to association with the object so that a mechanical event attendant the damage is capable of causing the triboluminescent means to emit light at least some of which is transmissible by the fiber optic means.
According to many embodiments of the present invention, xe2x80x9cdamage-autosensitivexe2x80x9d apparatus (e.g., apparatus which, in functional effect, is capable of automatically sensing damage to itself) is provided. The damage-autosensitive apparatus comprises a structure, at least one fiber optic line, and at least one triboluminescent element. Each fiber optic line is connectable to a photodetector and is situated so that a portion of the fiber optic line is in communication with the structure. Each triboluminescent element is integrated with the structure and is sufficiently proximate a fiber optic line so that, upon an occurrence of damage to the structure: (i) an accompanying mechanical action upon the triboluminescent element results in a luminescent emission of light by the triboluminescent element; and, (ii) at least a portion of the luminescently emitted light is transmissible to the photodetector via the fiber optic line.
According to frequent inventive practice, a method for sensing mechanical damage comprises triboluminescently radiating light in response to the damage, and fiber optically conveying at least some of the triboluminescently radiated light so as be informative about the mechanical damage.
Many inventive embodiments provide a method of sensing the damage condition of an object. The method comprises (a) integrating triboluminescent material with the object, and (b) associating at least one fiber optic line with the object and with a photosensitive device. The at least one fiber optic line is associated with the object and with a photosensitive device so that, following a damage-causing event accompanied by a mechanical action upon at least some of the integrated triboluminescent material, a quantity of a resultant triboluminescent light emanation is transmitted by at least one fiber optic line to the photosensitive device.
Featured by typical embodiments of the present invention is a sequence of events including a damage-related mechanical action with respect to triboluminescent material, followed by a triboluminescent emission of light, followed by a fiber optic admission of at least some of the emitted light, followed by a fiber optic transmission of at least some of the admitted light, followed by an electronic indication (e.g., including an identification, a registration, a recordation, a representation, a readout, a signal, a digitization, a processing and/or a display) of at least some of the transmitted light.
According to many inventive embodiments, the present invention""s fiber optic sensor senses fracture, delamination or similar mechanical damage in composites. The composite structure is doped with triboluminescent crystals, which release light upon fracture. The optical fiber then detects the light along its length, and transmits the light to a photodetectorxe2x80x94thereby indicating mechanical damage to the composite structure. The present invention thus affords a methodology for monitoring the physical condition of composite and other structures.
According to many embodiments of the present invention, a distributed fiber optic composite damage sensor utilizes triboluminescent material and optical fiber to detect mechanical damage (such as cracking or delamination) in a composite, and to transmit this information to a remote location. An important premise of typical embodiments of the present invention is that an optical fiber (sometimes referred to as an optical thread, line, fiber, filament or strand) can be embedded in a composite structure, andxe2x80x94anywhere along the optical fiber""s lengthxe2x80x94detect damage occurring within the composite structure, wherein the composite structure has been impregnated (e.g., in its matrix phase) with triboluminescent crystals. The present invention can be applied to any composite structure, including but not limited to edifices and other main structures, support structures, piping, propellers, hull sections and machinery components. The present invention can also be applied to non-composite structures meeting these and other descriptions.
The present invention admits of application to any structure which permits the association therewith (e.g., embedment therein) of a fiber optic line. The present invention is especially suitable for application to composite structures. The term xe2x80x9cfiber optic line,xe2x80x9d as used herein, refers to any discrete fiber optic member, such as an optical fiber, optical thread, optical filament, optical strand or a pluralized form thereof such as represented by a fiber optic cable. In accordance with the present invention, the association of a fiber optic line can be accomplished in any of several ways, such as by means of insertion of the fiber optic line after the structure has been made (for example, by machining the structure and adhesively bonding the fiber optic line in place), or by means of molding the fiber optic line into the structure during construction of the structure.
Fiber optic lines in general have certain qualities which are especially suited for inventive practice. They can be made to be relatively small in diameter and weight, yet relatively strong. They are immune to electromagnetic interference (EMI), and can provide distributed or multiplexed measurements. Moreover, fiber optic lines lend themselves to being embedded into rubbers, plastics, composites and even metals. Many inventive embodiments involve composite structures. In inventive application to fiber-reinforced matrix composites, one or more fiber optic lines can each serve as a reinforcement fiber in the fiber-matrix system of the structure.
Incorporated herein by reference are the following United States patents which disclose fiber optic technology: Cohen U.S. Pat. No. 6,080,982 issued Jun 27, 2000; E1-Sherif U.S. Pat. No. 5,060,307 issued Oct. 22, 1991; Rode et al U.S. Pat. No. 4,348,665 issued Sep. 7, 1982; Rouam U.S. Pat. No. 4,143,319 issued Mar. 6, 1979; Schutz et al. U.S. Pat. No. 4,509,364 issued Apr. 9, 1985; Jensen U.S. Pat. No. 4,328,462 issued May 4, 1982; Purvis et al. U.S. Pat. No. 4,655,077 issued Apr. 7, 1987; Riegler et al. U.S. Pat. No. 4,092,053 issued May 30, 1978; Slough U.S. Pat. No. 4,107,603 issued Aug. 15, 1978; Considine U.S. Pat. No. 3,981,621 issued Sep. 21, 1976; Fukuyoshi et al. U.S. Pat. No. 5,258,930 issued Nov. 2, 1993; Satake et al. U.S. Pat. No. 4,884,434 issued Dec. 5, 1989; Uejio U.S. Pat. No. 5,015,859 issued May 14, 1991.
Various fiber optic sensor technologies involving structural analysis or wear determination have been known or considered. Fiber optic sensor types which have been embedded for structural analysis include: fiber Bragg grating; long period grating; micro-bend interferometer; Fabry-Periot interferometer. Fiber optic measurement of strain, temperature, pressure, torque, vibration and acoustic fields have all been demonstrated. Structural health monitoring systems employing sacrificial, embedded optical filaments have been proposed for some time, primarily for composite structures. For instance, a fiber optic filament can be embedded into a structure in such a way that, if during its life cycle the loading exceeds the strength at the location of attachment, the optical filament breaks and the excessive load condition can be detected. These and more complex embedded sensor systems are now in use on air and space vehicles and civilian structures. See Udd, Eric, Fiber Optic Smart Structures, John Wiley and Sons, Inc., New York, 1995, incorporated herein by reference.
A composite is a combination of two or more materials which differ at the macroscopic level, each different material being a constituent of the composite. The following two references, incorporated herein by reference, are instructive regarding composites: John W. Weeton, Dean M. Peters and Karyn L. Thomas, Engineers"" Guide to Composite Materials, American Society for Metals, Metals Park, Ohio, 1987 (See, esp., Section 1, entitled xe2x80x9cIntroduction to Composite Materialsxe2x80x9d); George Lubin, Handbook of Composites, Van Nostrand Reinhold Company, New York, 1982 (See, esp., Chapter 1, entitled xe2x80x9cAn Overview of Compositesxe2x80x9d).
A matrix composite comprises (i) a filler or reinforcing agent (e.g., fibers, flakes or particles) and (ii) a matrix binder (e.g., a resin). The matrix is the principal phase or aggregate in which the filler or reinforcing agent is embedded or surrounded. Generally, the matrix serves two functions, viz., (i) it holds the reinforcement phase in place, and (ii) under an applied force, it deforms and distributes the stress to the reinforcement constituents. Types of matrix composite materials include metal matrix composite (MMC) materials, ceramic matrix composite (CMC) materials, and polymeric matrix composite (PMC) materials.
Examples of metals (metal elements, or alloys of two or more metal elements) conventionally used as matrices in metal matrix composites are aluminum, titanium, bronze and magnesium. A broad range of fillers or reinforcing agents (e.g, fibers) can be used with lower-melting point matrices in metal matrix composites. For instance, most metals, ceramics and compounds can be used as fillers or reinforcing agents in an aluminum or magnesium matrix. The choice of fillers or reinforcing agents for metal matrix composites becomes increasingly limited as the melting point of the metal matrix material increases.
Ceramic compounds (such as those used as matrices in ceramic matrix composites) are formed by the combination of one or more metallic elements with one or more nonmetallic elements. Examples of ceramic materials include aluminum oxide, magnesium oxide and silica.
There are two main types of polymers, viz., thermoplastics and thermosets. Examples of thermoplastics which can be used as matrix resins include polycarbonate, polyethylene, polystyrene, polypropylene, polyamide, fluoropolymer, thermoplastic polyester, nylon, vinyl, acetal, polycarbonate, polyphenylene oxide, polyetheretherketone (PEEK), polyphenylene sulfide (PPS), polyetherketone ketone (PEKK) and polyetherketone (PEK). Examples of thermosets which can be used as matrix resins include epoxy, polyester, vinyl ester, phenolic, polyimide and bismaleide.
Conventional types of fillers and reinforcing agents (e.g., reinforcing fibers or particles) used in the fabrication of matrix composites include glass, cotton, aramid, carbon, graphite, polyethylene, boron, steel, polyamide, alumina, silicon carbide and aluminaboria-silica.
A distinction may be drawn between the kinds of constituents in matrix composites which are xe2x80x9creinforcementsxe2x80x9d and those which are xe2x80x9cfillers.xe2x80x9d The primary function of a xe2x80x9creinforcementxe2x80x9d is to be a structural constituentxe2x80x94e.g., to afford strength and/or stiffness. For instance, in a fiber-reinforced plastic composite (FRP), the plastic functions as the matrix, while the fibers function mainly as reinforcements. In contrast, a xe2x80x9cfillerxe2x80x9d is a constituent which is added to a matrix for reasons other than structural (e.g., strength and/or stiffness), such as for increasing electrical conductivity, increasing thermal conductivity, improving fire resistance/retardance, cost reduction, control of shrinkage, increasing volume (e.g., acting as a bulking agent so that less resin is required) or other nonstructural purposes. Typically, fillers are particles that extend rather than reinforce the material. Such systems are generally referred to as xe2x80x9cfilledxe2x80x9d systems. Although the main purpose of fillers is not reinforcement, in some cases the filler may also, to some degree, serve to reinforce the matrix material.
Reinforcements and fillers may come in various forms. Fiber reinforcements are basically fibers, wherein their lengths are significantly greater than their diameters. Fiber reinforced composites have been classified as either xe2x80x9ccontinuousxe2x80x9d fiber composites or xe2x80x9cdiscontinuousxe2x80x9d fiber composites. Non-fiber reinforcements (and fillers) are characterized by dimensions that are roughly equal along all axes. Non-fiber reinforcements can be spheres, rods, plates, flakes and many other shapes. According to terminology adopted by many, the two categories of matrix reinforcement/filler shapes are (i) fiber and (ii) particle, wherein the term xe2x80x9cparticlexe2x80x9d is synonymous with the term xe2x80x9cnon-fiberxe2x80x9d and refers to any shape other than a fiber shape (which is characterized by length being considerably greater than diameter). The terms xe2x80x9cparticlexe2x80x9d and xe2x80x9cnon-fiberxe2x80x9d are used herein interchangeably to refer to any shape other than a fiber shape.
Generally, fibers are used as reinforcements; however, not all reinforcements are fibers, since particles are also commonly used as reinforcements. Although it is unusual for fibers to serve primarily as fillers, it is much more common for particles to serve primarily as fillers. Generally, matrix-containing constituents which are primarily intended for filler purposes are particles. On the other hand, matrix-containing constituents which are primarily intended for reinforcement purposes may be fibers or particles.
Diverse combinations of matrices along with fillers and/or reinforcements are possible within the basic matrix composite structure. For example, a matrix composite can include, within the same matrix, two or more kinds of reinforcing fibers. Additionally or alternatively, a matrix composite can include, within the same matrix, two or more kinds of reinforcements of various forms among fiber, particle, flake, etc. Additionally or alternatively, a matrix composite can include, within the same matrix, two or more kinds of fillers. Additionally or alternatively, a matrix composite can include, within the same matrix, both reinforcements and fillers. The term xe2x80x9chybridxe2x80x9d has been used to refer to these types of mixed filer and/or reinforcement schemes.
Matrix composites have one or more discontinuous phases surrounded by a three-dimensional continuous phase. There are also various types of non-matrix composites, which lack these attributes. For instance, felts and fabrics have no body matrix, consisting of different kinds of fibers held together by interweaving or entanglement, and perhaps together with a small amount of binder. Laminar composites are not matrix composites, but rather are characterized by flat, layered profiles. Examples of lamanar composites are a sandwich structure, plywood and a metal coated with another metal (e.g., to prevent corrosion). xe2x80x9cSandwichxe2x80x9d structures typically have a bulky, lightweight core which is situated between two thin, strong facings.
In accordance with the present invention, the triboluminescent crystals can be integrated with the composite structure in any of various ways or combinations of ways. For instance, according to many embodiments of the present invention, the present invention detects damage occurring within a matrix composite structure which has been impregnated with triboluminescent crystals in the matrix composite""s matrix phase. According to some inventive embodiments, the present invention detects damage occurring within a matrix composite which has been impregnated with triboluminescent crystals in its reinforcement phase and/or its filler phase. According to some inventive embodiments, the present invention detects damage occurring within a composite laminate structure which has been impregnated, within its lamina, with triboluminescent crystals.
In inventive practice, triboluminescent elements (e.g., triboluminescent crystals or polycrystals or aggregations thereof) can be integrated with the structure of interest in any of various ways in composites of all kinds, including fiber matrix composites, in particle matrix sit composites, reinforcement-plus-filler hybrid composites, fiber-plus-particle hybrid composites, composite laminates, or combinations thereof. For instance, triboluminescent elements can be, or be included in, some or all of the particle (and/or fiber) reinforcement constituents of a reinforcement-matrix system. Or, triboluminescent elements can be, or be included in, some or all of the particle filler constituents of a filler-matrix system. Or, triboluminescent elements can be, or be included in, some or all of the particle (and/or fiber) reinforcement constituents and/or some or all of the particle filler constituents of a reinforcement-plus-filler-matrix system. Or, triboluminescent elements can be mixed or blended or otherwise combined with the matrix material itself so as to constitute a fully integrated component of such matrix material. The triboluminescent elements can be distributed homogeneously (uniformly) or nonhomogeneously (nonuniformly) within the structure or structural portion with which they are united.
In accordance with the present invention, the triboluminescent elements can be generally distributed (e.g., in homogeneous fashion) at least substantially throughout the structure, or can be more specifically placed in one or more selected locations or zones of interest in the structure. The triboluminescent elements can be situated at or near a surface of the structure, or more deeply inside the structure. A key aspect of inventive practice is that the fiber optic line which is associated with the structurexe2x80x94more particularly, a light-permeable portion of the fiber optic linexe2x80x94be disposed in the vicinity of one or more triboluminescent elements; in other words, a light-permeable fiber optic portion must be positioned sufficiently near a triboluminescent element to permit a sufficient amount of luminescence-generated light to pass through its light-permeable axial exterior (outer covering, e.g., cladding) and reach its axial interior (centrally located optical fiber or fibers) in a sufficient amount or to a sufficient degree that its optical fiber(s) transmit an appreciable amount or degree of light indicative of the luminescence-generated light.
Advantageously, the present invention""s fiber optic composite damage sensor affords remote detection of damage on the surface of or within a composite structure or other structure such as a machinery component. The structure of interest can thereby be monitored for damage. The present invention is especially advantageous vis""-a-vis the current state-of-the-art because the present invention: (i) does not presuppose knowledge of the exact location of damage; (i) is a distributed sensor able to detect damage through great lengths; (iii) is a versatile sensor having great flexibility in reaching many areas within a composite or other structure.